BuildingFenceMedium

Post and Rail Fence

One 1,800 mm bay of post and rail fence: two rails bolted through the posts, no nail anywhere, so a rail that rots or a post that gets hit can be replaced without disturbing its neighbours.

Designthe dimensions everything else is derived from

Bay size
1800 mm post centres, 1200 mm above ground
Post
100 x 100 mm, 1800 mm long (600 mm buried)
Rails
2 per bay, 1700 x 100 x 45 mm, one at 1075 mm, one at 525 mm
Rail fixing
bolted through both post and rail
Exposure
fully external, ground contact at the post

Cut list sizes are finished sizes. Add 5 mm to the width and 25 mm to the length of every solid-wood part for saw kerf and planing, and buy boards oversize by one thickness so a bowed board can be flattened rather than relegated.

Materialswhat each part is made from, and why

All materials
Larch · 1 post, 2 rails per bay
Larch heartwood is durable without treatment and is grown in the same climates it is used in. It also holds a bolt hole better than a softwood that has been treated only at the surface.
Stainless Steel A4 (316) · every bolt, washer and nut
A fence is the least accessible thing anyone builds. Replacing one rusted bolt means digging out a post, so the fastener has to outlast the timber.

Cut listfinished sizes, worked back from the design

PartQtyFinished sizeStock
Post1100 x 100 x 1800 mmLarch
Rail21700 x 100 x 45 mmLarch

Fastenersquantities and lengths that follow from the joints

All standards
ItemQtyGripStandard
M10 x 160 hex bolt, A4 stainless8145 mmISO 4014
M10 plain washer16—DIN 125 A
M10 nylon-insert nut8—DIN 985

Grip is the sum of the clamped thicknesses. A bolt has to clear the nut by two full threads, which is what sets the length that appears in each row — not a preference.

M10 x 160 hex bolt, A4 stainless · grip 145 mm
Two per rail end, two rails, two ends. Grip is the post (100) plus the rail (45). The rail is 1700 mm because the bay is 1800 centres less one 100 mm post. ISO 4014 →
M10 plain washer
One under each head and each nut. DIN 125 A →
M10 nylon-insert nut
A fence rail is leaned on, climbed and shoved. The insert is what keeps it from rattling after two winters. DIN 985 →

Toolsand what each one is for on this job

All tools
Drill
Boring the 11 mm clearance holes through post and rail.
Twist Drill Bits
11 mm for M10 clearance, plus a long bit for boring through a 100 mm post in one pass from one side.
Circular Saw
Cutting the rails and the post to length.
Socket Set
Eight nuts, all at an awkward height.
Combination Wrench
Holding the head on the far face.
Tape Measure
Setting the two rail heights. Both rails level across a run of bays is the whole look of the fence.

Order of workthe sequence, and where it is not free to change

  1. Dig and set the post, checking it for plumb in two directions.
  2. Mark both rail heights on the post before fitting anything.
  3. Clamp a rail to the post and bore through both in one pass, so the holes are guaranteed to line up.
  4. Bolt up, snug, then bring to torque once the next post is set and the rail is known to be level.
  5. Repeat for the second rail, and stagger the joints so no two adjacent bays share a bolt height.

Where it goes wrongthe three failures worth designing out

  • Nailing rails to the faces of posts instead of bolting through them. Nails in a 45 mm rail into a 100 mm post have very little to hold in, and the first wet winter loosens the whole run.
  • Setting the rails by eye per bay. One bay out of level makes every following bay look wrong, and level is the only thing this kind of fence has to get right.
  • Using a plain nut because it was in the box. Half a day per bay is spent setting posts, and re-tightening a fence is a job nobody ever does.

Frequently asked questions

Why bolt rather than screw?

Because a bolt can be undone. A fence post is the single most likely thing on a property to be hit by a vehicle or a mower, and a bolted rail can be taken off and put back in ten minutes. This database does not carry wood screw standards, so no screw size is quoted here.

Should the rails sit between the posts or on the outside faces?

Between them, as this project does, so the bolts pass through post and rail both and the nut is on the inner face. Rails on the outside face would give a different look but the same joint, and the fence would be 90 mm wider for no gain.

How deep should the post be buried?

One third of its length is the usual starting point, which is what gives the 600 mm in a 1800 mm post. The failure mode this guards against is the post rotating in the ground under wind load, not the post breaking.